Dielectric heater electrode systems



June 30, 1959 G. E. GARD ET AL 2,892,915

DIELECTRIC HEATER ELECTRODE SYSTEMS Filed June 19, 1957 Q- Meier Generator FIG. 2.

To G enermor INVENTORS L. .MNNV 4 Werner R uegge berg pp George E Gard FIG; 5 ELL-Z4 ATTORNEY United States atent DIELECTRIC HEATER ELECTRODE SYSTEMS George E. Gard, East Hempfield Township, Lancaster County, and Werner Rueggeberg, Lancaster, Pa., assignors to Armstrong Cork Company, Lancaster, Pa., a corporation of Pennsylvania Application June 19, 1957, Serial No. 666,684

22 Claims. (Cl. 21910.67)

The present invention relates to electrode systems finding particular utility in dielectric heating applications; and is primarily concerned with an improved arrangement of electrode and reactive components which effectively employs stray electrode-to-ground capacities heretofore considered an undesirable feature of such electrode systems in producing voltage balanced push-pull electrode operation.

As is well known to those skilled in the art, dielectric heating applicators may take the form of a pair of electrodes spaced from one another and adapted to receive therebetween a load comprising dielectric material to be heated. Such electrodes may be disposed within or adjacent to a highly conductive shielding structure which is generally grounded, and the said electrodes themselves may be energized at an elevated voltage with respect to the grounded conductive enclosure or shielding structure by an appropriate source of oscillations.

The forms of electrode energization utilized in the past have, in general, taken two basic configurations. One such configuration, which will be termed hereinafter a single ended system, utilizes an oscillator having a single ended output coupled to one of a pair of electrodes, with the other electrode being directly connected to ground by an appropriate conductive strap. Such single ended systems, while of use in various drying applications, exhibit a number of disadvantages detracting from the effectiveness thereof. By way of example, the dielectric load normally present in prior art systems, including the aforementioned single ended systems, presents a high resistive impedance; and accordingly, rather complex impedance matching circuits are ordinarily required to effect impedance matching, and these matching circuits generally function to reduce the effective resistance of the load as seen by the aforementioned oscillator. In the case of known single ended systems, the impedance matching circuits required are, in general, more elaborate than in other electrode systems. In addition, such single ended systems by their very nature tend to radiate excessively whereby the use of such systems is often accompanied by a problem of appropriate shielding.

Moreover, in certain cases, such single ended systems cannot be employed at all when at least a portion of the dielectric heating or drying function relies upon the presence of stray fields existing between the electrodes and ground to produce a heating efi'ect. By way of example, it is sometimes desired to heat or dry stacked boards, such as the Travertone boards manufactured by Armstrong Cork Company at Beaver Falls, Pa.; and the heating of such stacked boards requires that the electrodes produce a direct field therebetween which is primarily useful in drying the centers of the boards, and that the electrodes also produce stray fields between each of the said electrodes and the grounded shielding structure or enclosure for heating or drying the edges of the boards. The direct grounded connection of one electrode in the single ended system precludes the existence of stray fields for edge heating adjacent such grounded electrode, whereby such a single ended system cannot be effectively employed in the drying arrangement mentioned previously.

In an attempt to obviate this latter known disadvantage of single ended drying systems (i.e. lack of stray fields), it has been suggested that a push-pull arrangement be employed; and such known systems comprise the second basic system employed heretofore, and will be termed hereinafter conventional push-pull systems. In general, such prior push-pull systems have employed conventional air core transformers having the primary winding coupled to the output of a single ended source and having the opposing ends of the secondary coupled respectively to two spaced electrodes. Such conventional push-pull systems do provide stray electrode-to-grou'nd fields of the type desired in effecting edge drying, and also tend to radiate somewhat less than the single ended systems discussed previously. However, such conventional push-pull systems in turn exhibit a number of appreciable disadvantages. In particular, they have no effect upon the highly resistive characteristic of the load being dried, whereby once more, as was the case in the single ended systems discussed previously, rather complex impedance matching arrangements must be employed which generally function to reduce the effective resistance of the load as seen by the oscillator. In addition, such convcntional push-pull systems often involve rather complex problems of coupling energy from the said oscillator to the electrodes themselves; and the overall arrangement, including the aforementioned air core transformer, often includes excessive reactive circuitry thereby giving rise to the possibility of frequency skipping of the oscillator.

In general, therefore, the two basic prior art systems discussed above exhibit a number of distinct disadvantages inherent in the systems themselves, and these disadvantages include problems of radiation, coupling, impedance matching, complexity of adjustment, voltage balancing, and cost. The present invention serves to obviate all of these dificulties and provides a novel electrode arrangement which is extremely simple in configuration but which, by recognizing and effectively employing the stray electrode-to-housing (i.e. to ground) capacities ordinarily present in such systems, exhibits better operating characteristics than anything possible heretofore.

It is accordingly an object of the present invention to provide an improved voltage balanced electrode system finding particular utility in dielectric heating applications.

Another object of the present invention resides in the provision of a coupling arrangement for use in dielectric heating electrode systems, which arrangement effectively employs stray electrode-to-ground capacitances for circuit completion efiects.

Still another object of the present invention resides in the provision of an improved coupling arrangement for use in dielectric heating installations whereby the said electrodes may be energized from a single ended source, and said electrodes may nevertheless operate effectively in push-pull without the use of conventional push-pull circuitry.

A still further object of the present invention resides in the provision of an improved arrangement of impedances adapted to cooperate with stray electrode-to-ground capacitances normally present in dielectric heater electrode systems, thereby to effect a substantially -degree phase reversal between voltages existing on a pair of opposed electrodes in a manner which is more simple and efficient that has been possible in systems suggested heretofore.

A still further object of the present invention resides in the provision of an improved electrode arrangement wherein stray capacitances existing between a pair of electrodes and ground are efiectively employed to produce a voltage balancing effect on the said pair of electrodes, whereby substantially perfect equality (magni tude) of electrode voltages to ground is directly achieved. Still another object of the present invention resides in the provision of an improved coupling arrangement for use in electrode systems of the types employed in dielectrio heaters, which system automatically effects a substantially purely resistive input impedance to the applicator thereby reducing the possibility of reactive loads reflecting backto the energization source and modifying the operation thereof.

Still another object of the present invention resides in the provision of an improved coupling arrangement for use in dielectric heater electrode systems which automatically efiects a resistive impedance transformation; and which, in particular, effects such a transformation which is greater than two-to-one over single ended sys temsemployed heretofore and which is four-to-one over conventional push-pul1 systems employed heretofore.

Still another object of the present invention resides in the provision of an improved electrode system for use in dielectric heater applications which is extremely simple in construction, which is less costly to construct and maintain, which exhibits a lower resistive input than systems suggested heretofore, which automatically effects a purely resistive input, which is easier to adjust than systems suggested heretofore, which reduces radiation and impedance matching problems of the types encountered heretofore, and which automatically produces a voltage balance on the electrodes themselves.

In accomplishing the foregoing objects and advantages, the present invention contemplates the provision of a novel electrode system wherein stray electrode-to-ground capacitances are carefully and intentionally employed not only to provide circuit completion effects through said capacitances, but also to effect phase reversal properties of one electrode voltage with respect to the other. The systems of the present invention employ a pair of electrodes disposed within or adjacent to a grounded shielding structure or enclosure, and each such electrode exhibits a stray capacity-to-ground, acting as a ground return for the said electrodes. A source of oscillations is provided which is physically connected to one only of the aforementioned electrodes, and the electrodes in turn are further associated with inductor means so arranged that voltage balanced effective push-pull operation is directly achieved from the aforementioned single ended source without the use of conventional air core transformer circuitry.

The actual arrangement of inductor means depends, as will be described subsequently, upon the nature of the stray electrode-to-ground capacitances existing in a particular system. By way of example, and as will be described in greater detail subsequently, the said stray electrode-to-ground capacitances may be relatively large in comparison with the direct capacitances existing between the electrodes themselves; and under such circumstances, the auxiliary inductor means utilized in the present invention may comprise a single variable inductor interconnecting the spaced electrodes. In certain other arrangements, such as will be described, the stray electrode-to-ground capacitances may be small in comparison with the direct capacitances existing between the electrodes; and under these latter circumstances, the inductor means may comprise one or more inductors shunt-connected to the aforementioned stray capacitances between the electrodes and ground. By appropriate selection of the inductors employed and by giving proper attention to the disposition of such inductors in the overall system, the aforementioned extremely simple, voltage-balanced, push-pull arrangement can, as will be described, effectively be achieved.

The foregoing objects, advantages, construction and operation of the present invention will become more readily apparent from the following description and accompanying drawings, in which:

Figure 1 illustrates an improved voltage balanced electrode system constructed in accordance with a preferred embodiment of the present invention.

Figure 2 is a schematic diagram comprising the equivalent circuit of the arrangement shown in Figure 1.

Figure 3 is a vector diagram illustrating the operation of the arrangement and circuit shown in Figures 1 and 2, respectively.

Figure 4 is a schematic diagram comprising the equivalent circuit of conventional singleended systems utilized heretofore.

Figure 5 is a schematic diagram comprising the equivalent circuit of conventional push-pull systems utilized heretofore.

Figure 6 is the equivalent circuit of another coupling arrangement constructed in accordance with a further embodiment of the present invention wherein the stray electrode-to-ground capacitances are relatively small rather than relatively large with respect to the direct capacitance existing between the said electrodes; and

Figure 7 is a vector diagram illustrating the operation of the circuit shown in Figure 6.

Referring now to Figure 1, it will be seen that in accordance with the present invention, a dielectric heater may comprise a pair of electrodes 10 and 11 disposed within a grounded shielding enclosure 12 in substantially parallel relation to one another. The electrodes them selves may, of course, take various configurations and be of various sizes depending upon the particular heating or drying application involved. In the case of the Travertone drying arrangement mentioned previously, the electrodes 10 and 11 are in fact relatively massive and the dimensions thereof are in the order of 3 x 6 x 4" per electrode. These massive electrodes are, in accordance with the present invention, supported within the grounded shielding structure of enclosure 12 by appropriate insulator means (not shown) whereby the said electrodes exhibit appreciable stray capacities-to-ground, these stray capacities being represented in Figure 1 by the dotted configurations designated C. In actual practice, the said stray electrode-to-ground capacitanccs, when the electrodes are of the aforementioned massive dimensions, may be in the order of 240 ,u Lf. between each electrode 10 and 11 and the grounded enclosure 12.

A dielectric load to be dried is, in accordance with known practice, disposed or fed betweenthe electrodes 10 and 11; and when the said load comprises the afore mentioned Travertone boards, they may assume a stacked board configuration 13 between electrodes 10 and 11, with new boards to be dried being fed onto the top of the stack, as at 14, and dried boards being fed off the bottom of the stack, as at 15. This particular stacked arrangement and the feeding techniques described, do not per se form a portion of the present invention; and in fact comprise the subject matter of a copending application.

As mentioned previously, the pair of electrodes 10 and 11 could be energized in accordance with various arrangements. When a typical single ended arrangement of the type discussed above is employed, the electrode 11 might, for instance, be strapped directly to the grounded enclosure 12, whereupon energy could be applied from an appropriate source to electrode 10. It will be noted, however, that such a strapped arrangement would eliminate the possibility of stray fields between the electrode 11 and the grounded enclosure, thereby preventing the stacked boards 13 from being dried along the edges thereof adjacent electrode 11; and therefore, in the particular case of a stacked board arrangement such as illustrated in Figure 1, the single ended energization arrangement is not in fact practical.

In order to effect stray fields between both of electrodes 10 and 11, the said electrodes could, of course, be

energized by a conventional push-pull system wherein the secondary of a substantially conventional air core transformer has its opposing ends coupled respectively to the electrodes and 11; but, as mentioned previously, this particular arrangement has a number of appreciable disadvantages. Accordingly, the present invention contemplates the provision of a novel energization scheme for coupling energy to the two electrodes 10 and 11 in such a manner that voltage balance and voltage phase shift properties are achieved without the use of such conventional air core transformers.

In effecting this improved electrode arrangement, the present invention takes into direct consideration the stray electrode-to-ground capacitances C, and uses these stray capacitances as a portion of a reactive network which provides the desired voltage balance and phase reversal. In particular, an appropriate single ended source of energy comprising an oscillator or generator 16 may have its output coupled via a transmission line 17 through the grounded enclosure 12 to the electrode 10, and the only physical connection between the electrode system and the said generator 16 in fact comprises the interconnection of transmission line 17 and electrode 10, as illustrated in Figure 1. A relatively massive inductor, comprising a substantially U-shaped conductor 18 is also provided, and the opposing ends of the said inductor 18 are coupled respectively to electrodes 10 and 11, as illustrated. The said inductor 18 may, in addition, include a strap 19 which is movable adjacent a portion of the inductor 18, as illustrated at 20, whereby movement of the said strap 19 varies the inductance of inductor 18 for the purposes to be described.

The actual purpose of the several elements thus described in reference to Figure 1 may be more readily appreciated by an examination of the equivalent circuit shown in Figure 2 and the vector diagram of Figure 3. Thus, referring to Figure 2, it will be seen that the pair of electrodes 10 and .11 have been designated by the letters A and B corresponding to a similar designation in Figure l. The electrode A is coupled to ground by the stray capacity C and this stray capacity in fact comprises the sole ground return for electrode A. In addition, the equivalent circuit of Figure 2 has shown a resistor R disposed in parallel with the stray capacity C and the said resistor R in fact represents the loss component of the load 13 due to stray field heating of the edges of the said load 13. Similar considerations are present with respect to the electrode designated B (and corresponding to the electrode 11 in Figure 1); and in particular, it will be noted that this electrode is also coupled to ground via the stray capacity C existing between electrode 11 and the shielded enclosure 12; and the said stray capacity C is in turn shunted by a further resistor R again representing the loss component due to stray field heating of the edges of the board stack 13.

In addition to the foregoing components comprising the, equivalent circuit, it will be noted that the conductive electrodes 10 and 11, being in spaced relation to one another, exhibit a capacity therebetween which has been represented by the capacitor C and this capacitor C is in turn shunted by'a further resistor R which represents the loss component in load 13 due to direct field heating between the electrodes 10 and 11. The inductor 18 has also been illustrated in Figure 2 as the variable inductor L interconnecting electrodes A and B.

The configuration thus described in reference to Figure 2 produces a phase reversal approaching very nearly 180 degrees between the voltages appearing on electrodes A and B when inductor L is appropriately adjusted; and inasmuch as in practice the stray capacities-to-ground C and C are equal, the arrangement not only produces almost complete equality of voltage magnitudes on electrodes A and B with respect to ground but also produces a pure resistive input impedance at the tapping point of transmission line 17 to electrode 10. These particular considerations may be most readily appreciated by examination of the vector diagram shown in Figure 3. As illustrated therein, the voltage E comprising the single ended output of generator 16 which is applied to electrode A (i.e. electrode 10), has been illus trated as being disposed at substantially zero degrees for purposes of clarity. This applied voltage at electrode A causes a current 1 to flow through the stray capacity C to ground; and inasmuch as the capacity C is very large in relation to the direct capacity C existing between the electrodes A and B (as will be discussed subsequently), the current 1 is highly capacitive and in fact leads the voltage E, by an angle approaching degrees.

Upon application of the voltage E to electrode A, a voltage B which is substantially in phase with E,,, is produced across the electrodes A and B; and the said voltage E effects a current flow 1 through the parallel combination of L, C and R Due to the inclusion of inductor L, this current 1 is highly inductive whereby it lags the voltage E by an angle again approaching 90 degrees. The inductive current 1 flowing through the stray capacity C between electrode B and ground, develops a voltage E, which in turn lags the current 1 by an angle again approaching 90 degrees.

In actual practice, and particularly for the component values to be described subsequently, the Q of the circuit illustrated in Figure 2 is approximately 25 and the several resistive components mentioned previously can, for all practical purposes, be ignored whereby the overall arrangement is substantially purely reactive for voltage distribution computation purposes. It should be recognized, therefore, that the vector diagram of Figure 3 is in fact exaggerated in its representation of angles; and, under practical operating conditions, the several angles mentioned previously so nearly approach 90 degrees that the voltages E and E, are substantially 180 degrees out of phase with respect to one another (see Figure 3). Moreover, inasmuch as the stray capacities C and C between each of the electrodes 10 and 11 and ground are equal, the input impedance to the electrode system is purely resistive so long as the total effective impedance between electrodes 10 and 11 (or between A and B) is maintained inductive at a magnitude of reactance twice that associated with the capacitive reactance to ground of either electrodes 10 or 11. With the inductor 18 (or L) appropriately adjusted as previously described, the magnitudes of the voltages E and E are equal to one another and the arrangement illustrated in Figure l and described in reference to Figures 2 and 3, therefore not only produces the desired 180-degree phase shift between the voltages appearing on electrodes 10 and 11, but said arangement also automatically produces a balance in the voltage magnitudes appearing on the said electrodes 10 and 11.

The foregoing features of the present invention may possibly be more readily appreciated by a brief examination of actual parameter values measured in a practical embodiment of the invention. It has been found that, when massive electrodes of the types mentioned previously are disposed within a grounded enclosure, the shunt capacities C and C are equal to one another and have a magnitude of 240 ltflf. The generator 16 may in fact produce an output of 16.4 mc./s., and may operate near the kw. level at an electrode voltage (A to B) of 20,000 volts. At the aforementioned frequency, each of the capacitors C and C exhibits a capacitive reactance of 40 ohms. The direct capacity C as actually measured in the aforementioned system, has a magnitude of 10 ,u rf, and it will be noted that this capacity is in fact of a different order of magnitude (i.e. approximately & from the stray capacitors C and C At the aforementioned frequency of 16.4 mc./s., the direct capacity C exhibits a capacitive reactance of approximately 970 ohms. In order to effect the voltage balance considerations and phase shift properties described in reference to Figure 3., it has been found that the overall inductive reactance between electrodes and 11 should, at the frequency of the generator selected, possess a magnitude very nearly twice as large as that of the stray capacitive reactance of either electrode A or B to ground; and in actual practice, it has been found that the inductor L must have a value of .72 H, whereby the said inductance L exhibits an inductive reactance of 74 ohms at the selected frequency of 16.4 mc./s. Moreover, since the capacity C possesses a capacitive reactance of 970 ohms, the parallel combination of C and L (74 ohms reactance) provides a total inductive reactance of 80 ohms as required by previous explanation.

One further feature of the arrangement illustrated in Figure 1 and described in reference to Figures 2 and 3 should be noted. As will be appreciated by an examination of Figure 3, the currents I and I flowing from the circuit branches coupled to electrode A, are very nearly 180 degrees out of phase. The input impedance of the overall electrode arrangement shown in Figure 2 is there fore substantially purely resistive; and this characteristic of the circuit has in fact been illustrated in the vector diagram in Figure 3 by the small resistive component I which is in phase with the applied voltage B from generator 16. As a practical matter, it is not possible to determine with any certainty the ratio of stray field to direct field loss, i.e. it is impossible to determine the ratio of R, to R and accordingly, the in-phase current 1,. represents a grouped quantity that cannot easily be resolved into components. In actual practice, however, it has been found that when the voltage E is 10 kilovolts, the capacitive and inductive current I and I are each in the order of magnitude of 250 amperes; and (for the 100 kilowatt operation described previously) the inphase resistive current I is in the order of 10 amperes.

Summarizing, therefore, it should be noted that by the very simple arrangement shown in Figure 1, the voltages E, and E, appearing on the electrodes 10 and 11 are substantially 180 degrees out of phase and are equal in magnitude to one another. Moreover, the input impedance of the overall arrangement is substantially purely resistive, with this resistive input impedance being achieved automatically; and the resistive input feature thus accomplished eliminates excessive reactive loading upon the generator 16 with attendant problems of frequency skipping and the like, present heretofore. In addition, the out-of-phase energization and voltage balance described is accomplished in conjunction with a single ended energization source which is coupled directly to one of the electrodes, thereby eliminating the coupling problems and excessive reactive circuitry which are characteristic of prior systems employing air core transformers.

The circuit of Figure 1 is extremely simple and, moreover, lends itself to ready adjustment, thereby eliminating computation of the value of inductor required to effect the phase shift and voltage balance characteristics described previously. In particular, as noted above, the inductor 18 is intentionally made variable. Inasmuch as the circuit when properly set up exhibits a purely resistive input, this feature of the circuit may be directly employed in determining empirically the correct value of inductance to be employed. Thus, a Q'meter 21 (see Figure 1) may be coupled between electrode 10 and ground, and the strap 19 may thereafter be moved in an appropriate direction until the Q-meter shows that no reactive component of impedance is observed on the electrode A. When such a purely resistive input occurs, the desired phase shift and voltage balance properties are present and the overall circuit is set up to operate as described above.

A further feature of the arrangement shown in Figures 1 through 3 is that it automatically produces a resistive input impedance transformation. As has been mentioned, a load such as 13 normally presents a relatively high resistance to the generator 16; and prior single ended or conventional push-pull systems therefore necessarily utilize various impedance matching arrangements between the generator 16 and the electrode systems themselves to reduce the load resistance presented to the oscillator. These impedance matching networks, such as have been utilized heretofore, have of course generally increased the complexity and cost of the overall system; but such impedance matching problems are, for all practical purposes, substantially eliminated by the arrangement of Figures 1 and 2.

This particular feature of the present invention may perhaps be most readily appreciated by examining the equivalent circuits for single ended and conventional push-pull arrangements which have been shown in Figures 4 and 5 respectively. Thus, referring first to Figure 4 which represents a substantially conventional single ended system, it will be noted that the electrodes A and B again exhibit the direct capacity C and direct loss component R which was previously described in reference to Figure 2. The electrode A also includes a stray capacity C to ground as well as a loss compo nent R representing stray field loss between the said electrode A and ground. It should be noted, however, that due to the direct grounding of electrode B which is characteristic of single ended systems, there is no stray capacity between the electrode B to ground and no loss component between B and ground.

The conventional push-pull or voltage balanced system utilized heretofore is illustrated in Figure 5, and it will be noted that such an arrangement may comprise an air core transformer having a primary winding 30 coupled at 31 to a single ended source of oscillation, with the secondary 32 of the said air core transformer having its opposing ends coupled respectively to electrodes A and B The electrodes A and B as was the case in the arrangements of Figures 2 and 4, again exhibit a direct capacity C and a direct loss component R therebetween. The said electrodes A and B in addition, present stray capacities C and C to ground 33 (which may comprise a virtual ground), and also present stray field loss components R to ground.

The relative characteristics of the circuits illustrated and described in reference to Figures 2, 4 and 5 will be readily seen by setting up equations representing input and consumed power in the several systems. Thus, in the novel coupling arrangement comprising a preferred embodiment of the present invention, the voltage E, equals the voltage E and the voltage E equals 2E, equals 2E;,. The input power to the system may be represented by the expression:

wherein R represents the equivalent resistive load presented by the entire electrode system to the generator. Therefore, by equating the various losses occurring in the arrangement of Figure 2, it will be seen that:

Equation 1 above represents, as mentioned previously, the equivalent input resistance of the system shown in Figures 1 and 2.

In the case of the single ended system shown in Figure 4, a similar analysis is as follows:

E2 E2 E2 retrain. 2) R R wherein R represents the equivalent single ended input resistance presented to a generator such as 16.

Finally, by applying a similar analysis to the circuit of Figure 5, it will be seen that the following relationwherein R is the equivalent input resistance of the conventional push-pull electrode arrangement as presented to a generator such as 16.

Comparing Equations 1, 2 and 3 above, it will be seen that R is less than /2 R and, moreover, R is A the value of R Accordingly, the novel arrangement described in reference to Figures 1 through 3 automatically effects a resistive input impedance transformation thereby simplifying and often eliminating impedance matching problems as well as oscillator frequency skipping problems due to excessive reactive circuitry.

The novel circuit thus provided has, of course, relied upon the presence of stray capacities between electrodes and ground to provide both electrode-to-ground circuit completion and also to provide a portion of the reactive circuitry employed in effecting the desired phase shift and voltage balance considerations which are characteristic of the circuit. In the particular arrangement of Figure 1, it has been assumed that the stray electrode-toground capacities are very high in comparison with the K 7 direct capacity existing between electrodes. Under some do circumstances, however (cg. the electrodes are widely spaced from the housing, or the said electrodes are very close to each other), an opposite consideration may be present, that is, the stray electrode-to-ground capacity may be relatively small in comparison with the direct capacitance existing between the electrodes.

The concepts of the present invention may nevertheless be applied in this further situation, and the equivalent circuit of Figure 6 and vector diagram of Figure 7' illustrate the characteristics of such a further arrangement. Thus, referring to Figure 6,. it will be seen that the electrodes A and B may again possess stray capacitances C and C between their surfaces and ground. In the particular circuit of Figure 6, the resistors R have been eliminated (compare with Figure 2), inasmuch as the stray capacitors C and C are assumed to be fairly small in comparison with the direct capacitor C whereby little if' any stray fields and losses due thereto exist between the electrodes A -B and ground. As before, the equivalent circuit includes a capacitor C representing the direct capacitance between the electrodes A and B and the circuit also includes a resistor R representing the direct loss component existing between the said electrodes A and 13 It should moreover be noted that, in the arrangement of Figure 6, the inductor utilized to efiect phase shift and voltage balance is connected between the electrode B and ground rather than between the two electrodes, as was the case in the arrangements of Figures 1 and 2. This different arrangement of inductor (represented in Figure 6 as X is of course necessitated by practical feasibility because of the different relative values of stray to direct capacitance which have been assumed; and as a practical matter, it has. been found and may be developed mathematically, that the desired voltage balance between electrodes A and B is realized when the combined impedance of X and X is maintained inductive at a value equal to one-half of the magnitude of X The actual voltage considerations for the circuit of Figure 6 may be readily appreciated by examination of the vector diagram in Figure 7. The voltage E which again represents the generator voltage applied this time to electrode A has been illustrated in Figure 7 as a vector lying at substantially zero degrees. This applied voltage effects a capacitive current flow through the stray capacity C which has been represented as the current 1 leading the voltage E by substantially degrees. The applied voltage E also produces a voltage E between electrodes A and B which is substantially in phase with voltage E,,, and this voltage E effects a current 1.; which leads the voltage E by substantially 90 degrees. Current I in passing through the parallel combination of X and C (which, as mentioned previously, is maintained inductive), develops a a voltage E at electrode B which leads the current I again by substantially 90 degrees.

As was the case in the vector diagram of Figure 3, the several vector relationships have been exaggerated in Figure 7 for purposes of clarity; and as a practical matter, the voltages E, and B will again be equal in magnitude and substantially degrees out of phase (see Figure 7). One pronounced difference may be readily seen, however, by comparison of Figures 3 and 7. While the input impedance of the arrangement described in ref erence to Figure 2 was substantially purely resistive, the input impedance of the arrangement described in reference to Figure 6 is highly capacitive; and the circuit will in fact draw a reactive input current comprising the vector sum of the capacitive current I and I This particular characteristic of the circuit shown in Figure 6 may, however, be readily corrected by use of a shunt inductive reactance 4.0 connected between electrode A and ground; and this inductive reactance 40, by proper adjustment, may be caused to draw a current substantially 180 degrees out of phase with the vector sum of the capacitive. currents I and 1., whereby the input impedance of the overall network is again substantially purely resistive. The overall arrangement thus effects the desired voltage balance and phase shift properties which were characteristic of the arrangement previously described in reference to Figure 2; and the circuit of Figure 6 in fact provides downward input impedance transformations similar to those which characterized the circuit of Figure 2.

One further feature of the several systems thus described should be noted. The circuit of Figures 1 and 2, which utilizes stray capacitances-to-ground which are relatively high in comparison with the direct capacitance between electrodes, is in fact quite stable in operation; and it has been found that the desired voltage balance and phase shift properties are maintained with a variatron of no more than :5% in voltage and no more than i10% in power, even though the dielectric constant of material being heated changes by as much as 1:30%. This possible variation in dielectric material is well. beyond practical reequirements whereby, under any application which might be encountered in practice, the structure and circuits of Figures 1 and 2 operate in an extremely efficient and stable manner.

It has been found, however, that the arrangement of Figure 6 is quite sensitive to dielectric constant changes; and the effective composite dielectric constant of the space between eletrodes, including air gaps, should be maintained within i3% of a chosen mean value in order for the power delivery to be maintained within i10%- of the desired level. This type of tolerance of dielectric constant is seldom realized in a static load, but is quite often realized when the load takes the form of an endless type of continuously moving load. Moreover, although i-3% changes of composite dielectric constant may be tolerable, actual material (heating load) dielectric constant changes may be considerably greater depending on the ratio of air gap length to thickness of the load.

Accordingly, while the arrangement of Figure 1 finds ready utility in the case of both static and continuously moving loads, the arrangement of Figure 6 is especially applicable in those cases wherein load consistency is achieved due to continuous movement of the load. It should, in addition, he noted that if the natural behavior of a moving load lacks the desired electrical properties to effect consistent power delivery in the arrangement of Figure 6, the said Figure 6 arrangement may nevertheless be employed if the rate of travel of the load is automatically controlled thereby to so vary the load presented to the oscillator that the output of the said oscillator is maintained substantially constant. One possible such arrangement for producing a substantially constant oscillator power output by varying the feed rate of loads, is described in a prior copending application of Werner Rueggeberg, Serial No. 658,174, filed May 9, 1957, for: .Deviation Amplifier and Control System.

While preferred embodiments of the present invention have thus been described, many variations will be suggested to those skilled in the art; and it must therefore be understood that the foregoing description is meant to be illustrative only and should not be considered limitative of the present invention. All such variations and modifications as are in accord with the principles described are meant to fall Within the scope of the appended claims:

Having thus described our invention, we claim:

1. In a dielectric heating system, a voltage balanced effectively push-pull electrode system comprising first and second spaced electrodes disposed in substantially parallel relation to one another for receiving a load to be heated therebetween, a grounded shielding enclosure adjacent said electrodes, said electrode system including stray capacitances between each of said electrodes and said shielding enclosure, said electrodes being supported in freely spaced relationship to said grounded enclosure whereby said stray capacitances act as ground return paths between each of said electrodes and said enclosure, a single ended oscillatory power source having an output connected to said first electrode only, and inductance means connected to said first and second electrodes for balancing the reactive components exhibited by said stray capacitances to said power source whereby said electrode system comprises a substantially purely resistive load on said source, the reactive impedance between said first and second electrodes being substantially larger than the reactive impedance between either of said first and second electrodes and ground at the frequency of said source.

2. In a dielectric heating system, a voltage balanced effectively push-pull electrode system comprising first and second spaced electrodes disposed in substantially par allel relation to one another for receiving a load to be heated therebetween, a grounded shielding enclosure adjacent said electrodes, said electrode system including stray capacitances between each of said electrodes and said shielding enclosure, said electrodes being supported in freely spaced relationship to said grounded enclosure whereby said stray capacitances act as ground return paths between each of said electrodes and said enclosure, a single ended oscillatory power source having an output connected to said first electrode only, and inductance means connected to said first and second electrodes for balancing the reactive components exhibited by said stray capacitances to said power source whereby said electrode system comprises a substantially purely resistive load on said source, the reactive impedance between said first and second electrodes being substantially larger than the reactive impedance between either of said first and second electrodes and ground at the frequency of said source, said inductance means comprising an inductor connected between said pair of spaced electrodes.

3. In a dielectric heating system, a voltage balanced effectively push-pull electrode system comprising first and second spaced electrodes disposed in substantially parallel relation to one another for receiving a load to be heated therebetween, a grounded shielding enclosure adjacent said electrodes, said electrode system including stray capacitances between each of said electrodes and said shielding enclosure, said electrodes being supported in freely spaced relationship to said grounded enclosure whereby said stray capacitances act as ground return paths between each of said electrodes and said enclosure, a single ended oscillatory power source having an output connected to said first electrode only, and inductance means connected to said first and second electrodes for balancing the reactive components exhibited by said stray capacitances to said power source whereby said electrode system comprises a substantially purely resistive load on said source, the reactive impedance between said first and second electrodes being substantially larger than the reactive impedance between either of said first and second electrodes and ground at the frequency of said source, said inductance means comprising a pair of inductors connected respectively between said pair of electrodes and ground.

4. In combination, a pair of conductive electrodes spaced from one another and exhibiting an effective direct capacitance therebetween, a grounded structure adjacent to but spaced from each of said electrodes, said electrodes each exhibiting a stray capacitance to said grounded structure having a magnitude different from that of said effective direct capacitance, a single ended source of oscillations having an output connected to one only of said electrodes, and an inductor coupled to at least the other one of said electrodes, the inductive reactance of said inductor, at the frequency of oscillations from said source, having a magnitude related to the magnitude of capacitive reactance of said direct and stray capacitances at said frequency such that there is a substantially -degree phase reversal between the voltages on said pair of electrodes.

5. In a dielectric heating system, an electrode system comprising a pair of spaced electrodes, a grounded structure adjacent to but spaced from each of said electrodes, said electrode system including stray capacitances between each of said electrodes and said grounded structure whereby said stray capacitances act as ground return paths between each of said electrodes and said grounded structure, each of said stray capacitances having a magnitude appreciably larger than the effective direct capacitance exhibited by said electrode system between said pair of spaced electrodes, an oscillator having an output coupled to one only of said electrodes, and an inductor connected between said electrodes, the size of said inductor being such that its inductive reactance is substantially twice as large as the capacitive reactance of each of said stray electrode-to-ground capacitances at the output frequency of said oscillator.

6. The combination of claim 5 wherein said inductor includes means for for varying the inductance thereof.

7. In a dielectric heating system, an electrode system comprising a pair of spaced conductive electrodes, a grounded structure adjacent to but spaced from each of said electrodes, each of said electrodes exhibiting a stray capacitance to said grounded structure which is appreci' ably in excess of the effective direct capacitance between said spaced electrodes, said stray capacitances comprising the sole electrical connection between said electrodes and said grounded structure, a single ended oscillatory power source having an output connected to one only of said electrodes, and a variable inductor connected between said electrodes, the magnitude of said inductor being such that it produces a substantially ISO-degree phase reversal between the voltages on said pair of electrodes.

8. In combination, an electrode system comprising first and second spaced conductive electrodes, a further conductive structure adjacent to but spaced from each of said electrodes, each of said electrodes exhibiting a stray capacitance to said" further structure having an order of magnitude different from that of the efiective capacitance between said spaced electrodes, a single ended source of oscillations coupled to said first electrode only, and inductor means coupled to said first and second electrodes for producing a substantially ISO-degree phase reversal between the voltages on said first and second electrodes, said inductor means exhibiting an inductive reactance which substantially balances the capacitive reactance of said stray and direct capacitances at. the frequency of said source whereby said electrode system acts as a substantially purely resistive. load on said source.

9. The combination of claim 8 wherein said further conductive structure comprises a grounded shielding enclosure disposed adjacent to said electrodes whereby said stray capacitances act as ground returns for each of said electrodes.

10. The combination of claim 8 wherein said stray capacitances each have a magnitude appreciably larger than the magnitude of said effective capacitance between said electrodes, said inductor means comprising an inductor having its opposing ends connected to said first and second electrodes respectively.

11. The combination of claim 8 wherein said stray capacitances each have a magnitude appreciably smaller than the magnitude of said effective capacitance between said electrodes, said inductor means comprising first and second inductors respectively connected between said first and second electrodes and said further conductive structure.

12. In combination, an electrode system comprising first and second spaced conductive electrodes, a further conductive structure adjacent to but spaced from each of said electrodes, each of said electrodes exhibiting a stray capacitance to said further structure having a magnitude appreciably smaller than the magnitude of the effective capacitance between said spaced electrodes, a single ended source of oscillations coupled to said first electrode only, and inductor means coupled to said first and second electrodes for producing a substantially l80-degree phase reversal between the voltages on said first and second electrodes, said inductor means exhibiting an inductive reactance which substantially balances the capacitive reactance of said stray and direct capacitances at the frequency of said source whereby said electrode system acts as a substantially purely resistive load on said source, said inductor means comprising first and second inductors respectively connected between said first and second electrodes and said further conductive structure, the magnitude of said second inductor being such that the combined reactive impedance of said second inductor and the stray capacitance between said second electrode and said further structure is inductive and has a value equal substantially to one-half the value of the capacitive reactance between said electrodes at the frequency of said source.

13. In a dielectric heating system, a pair of massive electrodes spaced from one another for the reception of a dielectric load to be heated therebetween, a grounded conductive enclosure shielding said electrodes, each of said electrodes being spaced from said enclosure thereby to define stray electrode-to-ground capacitances having a magnitude appreciably in excess of the effective direct capacitance between said spaced electrodes, each of said electrodes being returned to ground Via said stray capacitances, a source of oscillations having an output connected to one only of said electrodes, and reactive impedance means connecting said one electrode to the other of said electrodes, said reactive impedance means having a value related to the impedance of said direct and stray capacitances at a preselected frequency of oscillations from said source for producing a 180-degree phase reversal in the voltages on said electrodes at said preselected frequency.

14. In a dielectric heating system, a pair of massive electrodes spaced from one another for the reception of a dielectric load to be heated therebetween, a grounded conductive enclosure shielding said electrodes, each of said electrodes being spaced from said enclosure thereby to define stray electrode-to-ground capacitances having a magnitude appreciably in excess of the eifective direct capacitance between said spaced electrodes, each of said electrodes being returned to ground via said stray capacitances, a source of oscillations having an output connected to one only of said electrodes, reactive impedance means connecting said one electrode to the other of said electrodes, said reactive impedance means having a value related to the impedance of said direct and stray capacitances at a preselected frequency of oscillations from said source for producing a -degree phase reversal in the voltages on said electrodes at said preselected frequency, said reactive impedance means including an inductor comprising an elongated conductor having the opposing ends thereof connected to said pair of electrodes respectively, said conductor including a U-shaped portion, and a conductive strap movably bridging the legs of said U-shaped portion whereby movement of said strap varies the inductance of said inductor.

15. The combination of claim 14 including means coupled between said one of said electrodes and ground for measuring the reactive components of impedance presented to said source of oscillations during movement of said strap.

16. A dielectric heating system comprising a pair of electrodes, a grounded shielding enclosure adjacent said electrodes, each of said electrodes being returned to ground via stray capacitances existing between said electrodes and said enclosure, a source of oscillations having its output connected to one only of said electrodes, variable reactive impedance means connecting said one electrode to the other of said electrodes whereby the magnitude of said impedance means may be varied to produce a ISO-degree phase reversal between the voltages on said electrodes, and means for determining the eifective reactive impedance between said one electrode and said enclosure during variation of said variable impedance means.

17. A voltage balanced dielectric heating system comprising a pair of electrodes, a grounded shielding structure adjacent said electrodes, each of said electrodes being returned to ground via stray capacitances existing between said electrodes and said grounded structure, means for applying an oscillating potential to one only of said electrodes, and means for producing voltage balance on and voltage phase reversal between said electrodes comprising an inductor connecting said one electrode to the other of said electrodes.

18. The combination of claim 17 including means for varying the effective inductive reactance of said inductor.

19. A dielectric heating system comprising a plurality of electrodes, a grounded shielding enclosure adjacent said electrodes, each of said electrodes being returned to ground via stray capacitances existing between said electrodes and said enclosure, an oscillator, conductor means connecting the output of said oscillator to one only of said plurality of electrodes, and means for controlling the relative phase of voltages appearing on said plurality of electrodes comprising variable inductor means coupled to each of said electrodes.

20. In combination, a grounded shielding structure, an electrode system comprising first and second conductive electrodes disposed in spaced relation to one another and to said grounded structure, a source of oscillations connected to said first electrode only, said electrode system including a first reactive circuit comprising stray electrode-to-ground capacitance between said first electrode and said spaced grounded structure, said electrode sys tem including a second reactive circuit comprising direct capacitance between said first and second spaced electrodes, said electrode system including a third reactive circuit comprising stray electrode-to-ground capacitance between said second electrode and said spaced grounded structure, one of said second and third reactive circuits further including inductance means having a magnitude such that the reactive impedance of said second reactive circuit is substantially twice as large as the reactive impedance of said third reactive circuit at the frequency of said source of oscillations.

21. An electrode system comprising a pair of spaced electrodes, a grounded shielding enclosure adjacent said electrodes, each of said electrodes being returned to ground via stray capacitances existing between said electrodes and said enclosure, an oscillator for energizing said electrodes, and reactive means coupled to at least one of said electrodes for modifying the reactive impedance of a portion of said electrode system, at the frequency of said oscillator, so that the reactive impedance between said electrodes is substantially twice the reactive impedance between one of said electrodes and ground.

22. A dielectric heating system comprising a pair of electrodes, a grounded structure adjacent to and spaced from both said electrodes, said electrodes each being returned to ground via stray capacitances existing between said electrodes and said grounded structure, a source of oscillations having its output connected to one only of said electrodes, and reactive impedance means connecting said one electrode to the other of said electrodes for transferring energy to said other electrode, the reactive impedance between said electrodes, at the frequency of said source, being larger than the reactive impedance between either of said electrodes and ground.

References Cited in the file of this patent UNITED STATES PATENTS 2,308,043 Bierwirth Ian. 12, 1943 2,666,129 Ellsworth Jan. 12, 1954 2,783,344 Warren Feb. 26, 1957 2,813,184 Rueggeberg Nov. 12, 1957 2,824,940 Kuhlmann Feb. 25, 1958 

